The present invention relates to data communications equipment and, more particularly, to the adjustment of the transmit signal level of the data communications equipment.
The ability of data communications equipment to dynamically adjust the signal level, or amplitude, of a transmitted signal in order to compensate for changes in the transfer characteristics of a communications channel is well known in the art. Generally, the data communications equipment, or endpoints, of a communications network communicate to each other via a protocol that allows for the adjustment of a transmitted signal level of a particular endpoint. For example, in cable television (CATV), U.S. Pat. No. 5,155,590, issued Oct. 13, 1992 to Beyers II, et al., and U.S. Pat. No. 4,816,825, issued Mar. 28, 1989, to Chan et al., describe techniques that allow the CATV head-end to dynamically, or automatically, adjust the signal level from any downstream CATV station. Similarly, U.S. Pat. No. 4,554,579 issued Nov. 19, 1985 to Citta describes a technique that allows a network controller of a local area network to dynamically adjust a signal level from a data communications equipment that is coupled to the local area network. Both of these approaches require that a data connection exist between the endpoints of any network.
In addition, there is also a need in some situations to automatically adjust a signal level because of the type of equipment that receives the signal. For example, in the area of cellular data communications, a cellular modem provides a data signal to a cellular transceiver, which further modulates the data signal for transmission over a cellular telephone network to an answering modem that is located somewhere within the Public Switched Telephone Network (PSTN). Unfortunately, the signal level requirements of a cellular transceiver may vary from manufacturer to manufacturer. As a result, current cellular modems, e.g., a modem that supports the MNP-10 protocol (an MNP-10 modem), use an adaptive transmit routine to automatically adjust the transmit signal level of the cellular modem to match the signal level provided by the cellular transceiver. Like the above-mentioned U.S. Patents, in order to perform this adaptive adjustment, an originating cellular modem must first establish a data connection with a PSTN modem that supports the same protocol as the cellular modem. This allows the cellular modem to receive special "modem-only" packets, i.e., packets that are not provided to data terminal equipment but are used to transmit information as between the cellular modem and the PSTN modem. In this case, the cellular modem dynamically adjusts the transmit signal level by monitoring a predefined modem-only packet transmitted by the PSTN modem.
However, there are some problems associated with the above-mentioned type of automatic adjustment that affect the user of the equipment. For example, if either modem does not support the same protocol, then no adjustment of the transmit signal level by the either modem takes place. Unfortunately, this may result in one modem applying a default signal that is too "hot" to the respective communications channel. A "hot" signal is one where the value of the transmit signal level is so high that the likelihood of some type of distortion in the transmitted signal increases. For example, if a modem is coupled via a short local loop to the PSTN, the transmit signal level of the modem may be too hot with the result that processing by the PSTN may then "clip" the data signal. This clipping introduces errors into the transmitted data stream.
The co-pending, commonly assigned U.S. patent application of R. E. Scott entitled "Fixed Transmit Levels for Modems Operating Over Cellular," Ser. No. 08/096,282, filed on Jul. 26 , 1993, discloses a technique for overcoming some of the above-mentioned shortcomings. This approach allows a user to manually adjust, i.e., control, the transmit signal level of a cellular modem, or a PSTN modem, and avoid the above-mentioned problems. As a result, there is no requirement that a data connection must exist between the modem endpoints, or that the modem endpoints must support the same protocol. Consequently, the user can independently adjust either modem's transmitted signal level so that it is not too hot and, therefore, no clipping of the transmitted data signal takes place. In addition, connect time is reduced since there is no transmission of predefined modem-only packets between the modem endpoints.